The mean deviation is a measure of the average distance between each data point and the mean of the data set. To find the mean deviation, follow these steps:

1. Find the mean (average) of the data set.

2. Find the absolute difference between each data point and the mean.

3. Calculate the mean of these absolute differences.

Given data set: 2, 4, 5, 9

1. Mean = (2 + 4 + 5 + 9) / 4 = 20 / 4 = 5

2. Absolute differences:

- Absolute difference from 2 to the mean (5): |2 - 5| = 3

- Absolute difference from 4 to the mean (5): |4 - 5| = 1

- Absolute difference from 5 to the mean (5): |5 - 5| = 0

- Absolute difference from 9 to the mean (5): |9 - 5| = 4

3. Mean of absolute differences = (3 + 1 + 0 + 4) / 4 = 8 / 4 = 2.